1. Field of the Invention
The present invention relates to a rack-driven auxiliary power steering apparatus, and more particularly to a rack-driven auxiliary power steering apparatus which prevents a ball nut and a nut pulley from idling due to a slip thereof and increases durability when the drive belt is operated by driving of a motor, and minimizes vibrations and noise transferred through the rack bar, the ball nut, and the nut pulley when the rack bar is slid while the nut pulley and the ball nut are rotating, thereby providing a comfortable steering feeling to the driver.
2. Description of the Prior Art
Hydraulic power steering apparatuses using a hydraulic power of a hydraulic pump have been generally used as auxiliary power steering apparatuses for a vehicle, but in recent years, electric power steering apparatuses using a motor are being gradually commonly used.
A general electric power steering apparatus includes a steering system extending from a steering wheel to two wheels, and an auxiliary power mechanism for supplying auxiliary steering power to the steering system.
The auxiliary power mechanism includes a torque sensor for detecting a steering torque applied to a steering wheel by the driver and outputting an electric signal proportional to the detected steering torque, an electronic control unit (ECU) for generating a control signal based on the electric signal transferred from the torque sensor, a motor for generating auxiliary steering power based on the control signal transferred from the electronic control unit, and a belt type power transmission unit for transmitting auxiliary power generated by the motor to a rack bar via a belt.
FIG. 1 is a partially sectional view schematically illustrating a rack-driven auxiliary power steering apparatus according to the related art, and FIG. 2 is a sectional view schematically illustrating a portion of the rack-driven auxiliary power steering apparatus according to the related art.
As illustrated in FIGS. 1 and 2, the rack-driven auxiliary power steering apparatus according to the related art includes a rack bar 140 extending in a transverse direction of the vehicle and having a rack gear at one side of an outer peripheral surface thereof, a pinion shaft 110 having a pinion gear engaged with the rack gear, a ball nut 210 engaged with an outer peripheral screw groove 145 via balls 220, a belt type power transmission unit 160 connecting a ball nut 210 and a motor shaft 155, and a motor 150.
The pinion shaft 110 is connected to a steering wheel through a steering shaft, and the rack bar 140 having the outer peripheral screw groove 145 of a predetermined length at one side of the outer peripheral surface thereof is installed within a rack housing 120.
The belt type power transmission unit 160 includes a motor pulley coupled to the motor shaft 155 and a nut pulley coupled to the ball nut 210, and transmits auxiliary steering power generated by the motor 150 in proportion to the steering torque applied to the steering wheel to the rack bar 140 through the ball nut 210.
The rack-driven auxiliary power steering apparatus according to the related art includes the rack bar 140 having the outer peripheral screw groove 145 on an outer peripheral surface thereof, the ball nut 210 having an intermediate path 215 along which balls 220 circulate, the balls 220 rolling while contacting the outer peripheral screw groove 145 of the rack bar 140 and the intermediate path 215, and an end cap 230 attached to an end of the ball nut 210.
In the rack-driven auxiliary power steering apparatus according to the related art, the rack bar 140 is slid as the balls 220 roll while contacting the outer peripheral screw groove 145 of the rack bar 140 and an inner peripheral screw groove of the ball nut 210.
However, according to the related art, idling occurs as a slip is generated between the ball nut and the nut pulley, deteriorating power transmission efficiency and making it difficult to accurately transmit an auxiliary steering power.
Further, noise and vibrations are generated in the rack bar, the ball nut, the nut pulley, the bearing, and the rack housing when the rack bar is slid while the ball nut is rotated, which noise and vibrations are transferred to other components, causing the coupled components to be released.
In addition, the transferred noise and vibrations are transferred to the driver together with vibrations and noise of other parts, deteriorating a steering feeling of the driver.